Plasticizers based on cyclohexane diesters were described in the patent literature at least as early as 1937, in U.S. Pat. No. 2,070,770. This class of plasticizers was described as being useful for nitrocellulose plasticization. The cyclohexane diester dibutyl hexahydrophthalate or alternatively called dibutyl cyclohexanediacid ester, sold under the brand name Hydropalate B and at one time available from Deutsche Hydrierwerke AG, is taught as a “gelatinizer for cellulose nitrate but not for cellulose acetate” in Plasticizers, by D. N. Buttrey, Leaver-Hume Press LTD, London, 1950, pp. 16-27.
U.S. Pat. No. 2,311,259 describes cyclohexanoates, including the cyclohexanol ester thereof. The problem with this product, a solid at room temperature, is that it is inefficient as a plasticizer (meaning it requires a large amount to provide a plasticizing effect), has poor low temperature flexibility, and is not a good viscosity depressant in plasticisols.
Very little additional information appeared on these types of plasticizers until around 1960, following the commercialization by Union Carbide of their cyclohexane diester of 2-ethyl hexanol plasticizer Flexol™ 55-CC. This plasticizer was alternatively described as di-2-ethylhexyl hexahydrophthalate, and it was reported as being useful in flexible PVC plasticization. This material, otherwise known as di-2-ethylhexyl cyclohexanediacid ester, is described in Industrial Plasticizers by Ibert Mellan, The Macmillan Company, New York, 1963, pages 190-191. Specifically, the synthesis of Flexol CC-55 is set forth and described as a primary plasticizer for vinyl chloride resins, and said to compare favorably with dioctyl phthalate (DOP) in many basic properties. The PVC plasticized thereby it said to be processable by calendering, extruding, and molding, and as a plastisol. It is also described as compatible with a number of polymers, including polyvinyl acetate, polyvinyl butyral, polystyrene, and chlorinated and brominated rubber.
In the late 1990's and early 2000's, BASF, ExxonMobil, and others described compositions based on cyclohexanoate, cyclohexanedioates, and cyclohexanepolyoate esters, said to be useful for a range of goods from semi-rigid to highly flexible materials. See, for instance, WO 99/32427, WO 2004/046078, WO 2003/029339, WO 2004/046078, U.S. Application No. 2006-0247461, and U.S. Pat. No. 7,297,738.
Phthalate ester plasticizers based on secondary alcohols have also been known since the 1950's. Once such product was di-capryl phthalate, mentioned in D. N. Buttney's text, supra. The problem with this product, among others, is that it is based on capryl alcohol (C8 alcohol with substitution almost exclusively at the 2-position), which is a by-product of sebacic acid production, and hence its availability is limited. This plasticizer was used, for instance, in polyvinyl chloride foam products with silicone polymers. See U.S. Pat. No. 3,795,634. Dicapryl phthalate or DCP is particularly known as a plasticizer giving low, stable viscosities in plastisol formulations versus viscosities that can be obtained with other esters. See, for instance, The Technology of Plasticizers, J. Kern Sears and Joseph Darby, John Wiley & Sons, 1982 p. 349.
Work in the 1960-70's confirmed that secondary alcohol phthalate esters are good plasticizers provided that the hydroxyl group is positioned predominately at the 2-position. See J. W. Hayden, Soc. Plastic. Eng., Annual Tech. Conference., Tech Papers, 28th (1970), pages 46-47 (SPE Publisher, Greenwich, Conn.).
Despite all the above-mentioned work, an efficient plasticizer that can be easily and inexpensively manufactured is still being sought.
The present inventors have surprisingly discovered that secondary alcohol esters of cyclohexanecarboxylic acids based on C7-C12 secondary alcohols, such as cyclohexanemonocarboxylic acids, and cyclohexanedicarboxylic acids, are efficient as plasticizers in PVC and other polymer systems, offering improvements over other plasticizers.